You assume culture is unchangeable, like race. Right now their higher fertility seems to be balanced out by people leaving them, and this has been true for a while.
Religiosity is around 30-50% heritable (compared to 50-80% for IQ), and social conservatism is around 40-60% heritable. If the leaver culture remains low fertility, the personality traits that encourage leaving will be selected against.
This comment doesn't really respond to the parent; heritability says nothing at all about mutability or whether transmission is cultural.
Assuming such traits exist. Fact is, living on a farm for the sole purpose of reproduction is a shit life.
Yet another person talking about inheritability without understanding what the word means.